Hydraulic control for a dividing machine tool

ABSTRACT

A hydraulic control for a dividing machine tool, in particular a shear or cutting press, in which a damping device is provided in order to counteract a sudden downward movement of the working piston upon the cutting through of a workpiece during an operating stroke.

FIELD AND BACKGROUND OF THE INVENTION

The present invention relates to a hydraulic control for a dividingmachine tool.

Such hydraulic controls are used, in particular, in shearing presses orcutting presses by which workpieces are brought to predeterminedlengths. Thus, for instance, scrap shears are known by which railwayrails can be divided into pieces of short length before melting orrecycling.

In such machine tools one distinguishes essentially between threeindividual strokes, during the first of which strokes (the closingstroke) the cutting tool is closed. This is then followed by the workingstroke, in which the work piece is cut in two, while a last strokeperforms the return movement, i.e. the opening of the tool in order tobe able to remove or shift the workpiece.

In order to prevent an uncontrolled lowering of the cutting tool, a backpressure acts on the working piston of the machine tool during theclosing stroke, it approximately compensating for the weight of theparts to be lowered.

In this way, assurance is had that the workpiece of the machine tooldoes not descend solely due to its own weight, so that the hydraulicpump must always operate against the back pressure and thus thehydraulic cylinder is always filled with hydraulic fluid.

During the working stroke, i.e. during the cutting of the workpiece, theback pressure is decreased so that the maximum cylinder force can beused for the actual cutting process.

Upon cutting the workpiece, the entire hydraulic pressure continues toact on the working piston, so that the latter is accelerated immediatelyafter the cutting until the hydraulic pressure decreases due to thecontrol of the machine tool and the return movement of the tool iscommenced. This sudden acceleration of the working piston can lead to atearing apart of the oil column on the pressure side in the workcylinder so that there is a sudden, uncontrolled movement of the workingpiston. This sudden change in speed of the work piston is also known asthe "cutting shock" and it can lead to damage to the drive and hydrauliccomponents of the machine tool.

SUMMARY OF THE INVENTION

In contradistinction to this, the object of the invention is to create ahydraulic control for a dividing machine tool with which, with a littleapparatus expense, uncontrolled movement of the working piston can beprevented.

According to the invention the measure of providing a damping device, adamping pressure which opposes the downward movement of the workingpiston can be applied at the moment that the workpiece is cut through,so that the oil column on the pressure side in the work cylinder doesnot tear apart and thus sudden load peaks are counteracted.

The damping device is advantageously provided with means for limitingthe maximum pressure so that hydraulic fluid can be expanded into ahydraulic tank when a maximum system pressure is exceeded.

A particularly simple control of this pressure-limiting valve device isobtained if, on the control side thereof, a predetermined controlpressure is applied during the closing stroke which produces a backpressure which becomes inactive when an adjustable work level is reachedand which can have a damping pressure superimposed on it or be replacedby a damping pressure which is greater than the control pressure. Thus,during the closing stroke, a back pressure acts which becomes inactiveupon the work stroke when the work level is reached and then, upon theparting, increases to a higher damping-pressure level so that thecutting shock is reduced.

In accordance with an advantageous further development of the invention,the expansion control pressure can be taken from the feed line to thework cylinder, while the control pressure during the closing stroke isadvantageously obtained from the return line.

The pressure in the feed line can then advantageously be fed by a bypassline into the control circuit of the pressure-limiting valve device. Inthis connection, a non-return valve and a flow valve, for instance anozzle, can be arranged in series with one another in the bypass line.

In a further development of the invention, the control pressure isbranched off to the work cylinder from the return line via another flowvalve, for instance a nozzle, and the bypass line is allowed to debouchdownstream of the flow valve into the control circuit.

Other advantageous embodiments of the invention are provided herein.

BRIEF DESCRIPTION OF THE DRAWINGS

One embodiment of the invention will be explained in further detailbelow with reference to a diagrammatic drawing in which:

FIG. 1 is a circuit diagram of a hydraulic control having a damping inaccordance with the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The FIGURE shows a circuit diagram of a hydraulic control for a scrapshear in accordance with the invention. In this connection, a workingpiston 2 guided in a work cylinder 4 is supplied with hydraulic fluid bymeans of a hydraulic pump 6, each of the two cylinder chambers 10, 12being adapted to be connected with a line conducting the pump pressure Por with a line leading to the tank T via a directional valve or similarcontrol means, in particular a proportional valve 8.

For this purpose, in the embodiment shown the proportional valve 8 isdeveloped as a pilot controlled 4/3-directional valve which can becontrolled via electromagnets a,b and/or pilot control valves. In itsbasic position shown in the FIGURE, the valve slide is urged into itsneutral position N by means of two springs.

From the hydraulic pump 6 a line section 14 leads to a connection P ofthe proportional valve 8. This connection P is connected in the neutralposition N to a connection T of the proportional valve 8, which, in itsturn, is connected via a tank-relief line 16 to the tank T.

From a branch 17 of the line section 14, a branch line 14a branches offin which there is connected a pilot controlled pressure-limiting valvethe opening pressure of which is adjustable. The pressure prevailing inthe branch line 14a is conducted via a control pressure line 20 to thecontrol side of the pressure-limiting valve 18 so that, when a maximumpressure is exceeded, a connection to the tank T can be made via thepressure-limiting valve 18, and the pump conveys the hydraulic fluidinto the tank T until the increase in pressure has been done away with.

From the outlet side of the proportional valve 8, a work line 22(hereinafter referred to as return line 22) extends from a connection Ato the lower cylinder chamber 12 in the FIGURE. Another work line 24 (inthe following referred to as the feed line 24) leads from a connection Bof the proportional valve 8 to the upper cylinder chamber 10 in theFIGURE of the working cylinder 4.

The working piston 2 is developed as a differential piston, the weightof the working piston 2 and of the tool indicated by G in the FIGURE.

In the neutral position N shown, the connections A and B are blockedwhile--as already mentioned--the connections T and P are connected toeach other so that the hydraulic fluid is circulated into the tank T inthe neutral position N (constant pump).

In the lift position, designated H in the FIGURE, of the valve slide ofthe proportional valve 8, the connections A, P and B, T respectively areconnected to each other, so that the hydraulic fluid is conveyed via theline section 14, the connections P, A, and the return line 22 to thepiston-rod-side cylinder chamber 12, while the hydraulic fluid can flowout of the piston-side cylinder chamber 10 via the feed line 24, theconnections B, T, and the tank-relief line 16 into the tank T. In thisway, the working piston 2 moves upward in the FIGURE, so that the toolof the machine tool moves away from the workpiece W and the scrap shearis opened.

In the "lower" position S of the valve slide, the two connections A andT are closed off, while the connections P and B are connected to eachother so that hydraulic fluid is conveyed via the line section 19, theconnections P, B, and the feed line 24 into the piston-side cylinderchamber 10 in order to lower the working piston 2 (closing stroke). Inorder to avoid controlled lowering, a back pressure is built up in thepiston-rod-side cylinder chamber 12 which acts against the pump pressurein the feed line 24 and in the piston-side cylinder chamber 10.

The back pressure is limited via a valve device 26, which will bedescribed in further detail below, to a value which is about 10 to 20%higher than the load pressure produced by the weight G.

When the preset back pressure is exceeded, a back pressure line 30 whichbranches off from the return line 22 downstream of the proportionalvalve 8, is connected via a control chamber 31 of a precontrolled powerpart, or value element, 28 to a connecting line 32 which leads to thetank T so that the pressure in the piston-rod-side cylinder chamber 12can be broken down to the tank T and the preset back pressuremaintained. The controlling of the power part 28 is effected via thevalve device 26, the construction of which is explained below.

The valve device 26 which produces the control pressure for the controlof the power part 28 has three connections X, Y and Z2, of which theconnection X can be connected to the back pressure line 30 of the returnline 22 and the piston-rod-side cylinder chamber 12, the connection Ycan be connected to the tank T, and the connection Z2 can be connectedto the feed line 24 and thus to the piston-side cylinder chamber 10. Thecontrol pressure for the power part 28 is present on another connectionZ1.

A control line 34 which extends away from the inlet connection X leads,via a damping choke 36, to the input P of a pressure-controlled switchvalve 38. The latter is prestressed via an adjustable spring into aposition connecting the connection P with an output connection A (seeFIGURE). On the other control side of the pressure-controlled switchvalve 38, there is conducted a signal pressure line 40, so that, at apredetermined signal pressure, the valve slide of thepressure-controlled switch valve 38 can be displaced against the springbias pressure and the connection A of the pressure-controlled switchvalve 38 can be connected with a connection T from which a relief line22 leads to the connection Y of the valve device 26 and thus to the tankT. From the relief line 42, a signal pressure line 40 leads back to thespring prestressed control side.

From the output connection A of the pressure-controlled switch valve 38,a control-pressure line section 46 extends via a choke 46 to theconnection Z1 of the valve device 26 and thus to the control side of thepower part 28. In the region between the choke 46 and the connection A,a branch line 48 branches off, into which line there is switched amaximum-pressure limiting valve 50 by which the pressure in the branchline 48, and thus in the control-pressure line section 44 can berelieved via the connection Y into the tank T as soon as thepreadjustable maximum value for the control pressure in the controlcircuit has been reached.

Between the connection A and the branch line 48, another branch line 52branches off in which there is provided a back pressure limiting valve54. This back pressure limiting valve 54 is also preadjustable, so thatwhen a predetermined back pressure is exceeded, it is conducted via acontrol pressure line 56 branching off from the other branch line 52 tothe control side of the back pressure limiting valve 54 and in this waythe piston slide is brought into a position connecting the connections Pand T of the back pressure limiting valve 54 to each other, so that thepressure in the control circuit can be relieved via a switch valve 58,described in further detail below, and the connection Y into the tank Tof the hydraulic valve.

As already mentioned above, the back pressure is so adjusted that it isabout 10 to 20% above the load pressure produced by the weight G.Ordinarily, the back pressure thus lies in the range of 100 to 150 bar.The maximum pressure to be switched via the maximum-pressure limitingvalve 50 is substantially higher than this back pressure, which isessentially presettable via the back pressure limiting valve 54.

The above-mentioned switch valve 58 is developed as anelectromagnetically actuatable 4/2-way directional valve, the controlline section 60 leading from the connection T of the back pressurelimiting valve being conducted to a connection A of the switch valve.The control side of the pressure-controlled switch valve 38 is connectedvia the signal pressure line 40 to a connection B of the switch valve58.

The pressure in the piston-side cylinder chamber 10 is conducted via asignal pressure line 62 to the connection Z2 of the valve device 26 andfurther to a connection P of the switch valve 58. From a tank connectionT of the switch valve 58, a relief line 64 leads to the connection Y ofthe valve device 26.

In its neutral position N, the valve slide of the switch valve is urgedby spring bias pressure into a position in which the connections A and Pare blocked while the connections B and T are connected to each other,so that the control pressure in the upper control side in the FIGURE ofthe pressure-controlled switch valve 38 is reduced via the singlepressure line 40, the connections B and T of the switch valve 58, therelief line 64 and via the connection Y of the valve device 26 into thetank T so that the valve slide of the pressure-controlled switch valve38 is urged by spring action into the position thereof which connectsthe connections A and P of the pressure-controlled switch valve 38together.

In the position G of the valve slide, the connections P, B, on the onehand, and the connections A, T of the switch valve 58 on the other handare connected with each other. In this case, the pressure in thepiston-side cylinder chamber 10 is conducted over the signal pressureline 62, the connections P, B of the switch valve 58, and the signalpressure line 40 to the control side of the pressure controlled switchvalve 38, so that its control slide is brought against the spring biaspressure into the position thereof which connects the connections A andT with each other and in which the pressure in the control pressure linesection 14, and thus the pressure on the control side of the power part28, can be relieved to the tank T via the connections A, T of thepressure-controlled switch valve 38, the relief lines 42, 64 and theconnection Y. In other words, in this switch position, the controlpressure on the control side of the power part 28 is determined solelyby a mechanical prestressing device, such as, for instance, a spring 66.

From the signal pressure line 62, a bypass line 68 branches off, itdebauching into the control pressure line 34 between the choke 36 andthe pressure-controlled switch valve 38.

A choke 70 and non-return valve 72 are connected in series in the bypassline 68. By means of the non-return valve 72, a fluid flow from thecontrol pressure line 34 via the bypass line 68 to the signal pressureline 62 is prevented, while a flow in opposite direction upon theexceeding of a pressure predetermined by the spring bias tension of thenon-return valve 72 is possible. In this way, the pressure in thepiston-side cylinder chamber 10 can be fed via the bypass line 68 intothe control circuit for controlling the power part 28.

For a better understanding of the function, the switch processes duringthe closing stroke, the working stroke, and the return movement of theworking piston 2 will be explained below.

Lowering of the Working Piston 2 (Closing Stroke)

For the lowering of the working piston 2, the magnet a of theproportional valve 8 is provided with current so that the valve slide ofthe proportional valve 8 is displaced from its neutral position N intothe position S. In this position S, as already mentioned, theconnections P and B of the proportional valve 8 are connected to eachother, while the connections A and T are blocked. In this way, hydraulicfluid is pumped by the pump 6 into the piston-side cylinder chamber 10so that the piston is lowered. The lowering is effected against a backpressure in the piston-rod-side cylinder chamber 12 which is establishedvia the power part 28 and the valve device 26.

In this connection, the control pressure corresponding to the pressurein the cylinder chamber 12 is conducted via the connection X of thevalve device 26 to the connection P of the pressure-controlled switchvalve 38. Upon the downward movement of the working piston 2, the magneta of the switch valve 58 is also provided with current, so that thevalve slide of this valve is in the position in which the connections P,B and A, T respectively are connected to each other and the pressure inthe piston-side cylinder chamber 10 is present on the control side ofthe pressure-controlled switch valve 38, which pressure, however, isstill not sufficient to bring the valve slide of the pressure-controlledswitch valve 38 into its position connecting the connections A, T. As aresult, the connections A, P of the pressure-controlled switch valve 38are connected with each other so that the control pressure is conductedin the control pressure line 34, the connections A, P of thepressure-controlled switch valve 38, the control pressure line section44 and the choke 46 on the control side of the power part 28. The backpressure upon the lowering of the working piston 2 is then determined bythe back pressure limiting valve 54 the input connection P of which canbe connected via the tank connection T and the control-line section 60,the connections A, T of the switch valve 58, and the connection Y to thetank T if the back pressure exceeds a preset value.

Working Stroke (Cutting of the Workpiece)

As soon as the tool of the scrap shear comes against the workpiece W,the pressure in the piston-side cylinder chamber 10 increases greatly,whereby the control pressure in the signal pressure lines 40, 62 alsoincreases, so that the valve slide of the pressure-controlled switchvalve 38 is brought out of its position shown in the FIGURE, against thebias tension of the spring, into its position connecting the connectionsA and T, and the control pressure in the control-pressure line section36 can be relieved into the tank T via the relief lines 42, 64 and theconnection Y. In this way, the spring bias pressure of the power part 28is determined solely by the action of the spring 66 so that, as a resultof the comparatively low spring initial tension over the power part 28,the pressure in the back pressure line 30 is relieved into the tank T,and the back pressure decreases to zero and the full cylinder pressurecan be used for the cutting of the workpiece W.

Cutting Shock

Upon the cutting of the workpiece W, the full cylinder force acts on theworking piston 2 so that a back pressure must be built up in order toprevent the above-described cutting shock.

By the sudden elimination of the resistance applied by the workpiece,the pressure in the piston-side cylinder chamber 10 drops, as a resultof which the control pressure on the control side of thepressure-controlled switch valve 38 also decreases and the latter isagain moved into its basic position, shown in the FIGURE, in which theconnections A and P are connected to each other via valve slide sothat--as described above--the pressure prevailing in the piston-rod-sidecylinder chamber 12 can be built up as control pressure or as backpressure on the control side of the power part 28. The building-up ofthis back pressure takes place, however, too slowly to assure effectivedamping of the cutting shock. In order to accelerate the build-up ofpressure, the greater pressure in the piston-side cylinder chamber 10 isfed via the bypass line 68, the choke 70 and the non-return valve 72,into the control-pressure line 34, which pressure is conducted via theconnections P, A of the pressure-controlled switch valve 38 and thecontrol pressure line section 44, the choke 46 to the connection Z1 andthus to the control side of the power part 28, so that a sufficientpressure builds up very rapidly on the control side of the power part 28in order to block the connection between the back pressure line 30 andthe connecting line 32 to the tank T. By the action of the comparativelyhigh pressure in the piston-side cylinder chamber 10, the power part 28can thus be brought very rapidly into its closed position so that a backpressure can build up very rapidly in the piston-rod-side cylinderchamber 12.

By this measure, the cutting shock is effectively damped so that damageto and overloading of the parts of the scrap shear upon the cutting ofworkpieces is prevented.

Return Movement of the Working Piston 2

For the return movement of the working piston 2, the electromagnet b ofthe proportional valve 8 is provided with current so that its valveslide is brought into the H position in which the connections A, P andB, T respectively are connected to each other. Furthermore, theelectromagnet a of the switch valve 58 becomes without current so thatits valve slide is brought by the initial spring tension into theneutral position N shown in the FIGURE, in which the connections A and Pare blocked, while the connections B and T are connected to each other.In this way, the control side of the pressure-controlled switch valve 38is connected with the tank T so that its control slide remains in theposition connecting the connections A, P, solely as a result of thespring initial tension.

The section of the control line of the back pressure limiting valve 54which leads away from the tank connection T is blocked by the switchvalve 58 so that a higher pressure than the preset holding pressure canbe adjusted in the control pressure line section 44 and thus on thecontrol side of the power part 28. Since in this switch position, thepressure at the inlet of the power part 28 (line section 30) and at thecontrol side of the power part 28 are the same, the power part 28 isheld by the action of the spring 66 in its closed position in which theconnection between the back pressure line 30 and the connecting line 32is blocked off.

Thus, hydraulic fluid can be pumped by the pump via the connections P, Aand the return line 22 into the piston-rod-side cylinder chamber 12 sothat the working piston 2 is moved back into its initial position shownin the FIGURE. The hydraulic fluid displaced from the piston-sidecylinder chamber 10 is discharged via the feed line 24, the connectionsB, T of the proportional valve 8, and the tank relief line 16 into thetank T.

Of course, the circuit shown can be expanded so that, in order to handlemore complicated operating tasks, a control for the differentialdownward movement of the working piston can also be provided.

The method of the invention, thus, for the first time, provides aneffective cutting shock damping which can be produced in simple mannerand without fundamental changes in the control block.

We claim:
 1. A hydraulic control for a dividing machine tool, suitablefor a shear or cutting press, a work cylinder of which can be controlledvia a valve device so that hydraulic fluid can be fed to a work pistonfor a work stroke, for a forward closing stroke and for return movementaway from a workpiece, wherein a back pressure valve device is providedin order to control a closing stroke of the work piston against apredetermined back pressure, in the manner that a connection is openedand closed between a tank of the hydraulic system and a return line fromthe work cylinder, wherein there is provided a damping device with apressure limiting valve device (26), the work cylinder (4) having afirst, piston side, chamber (10), and the hydraulic control furthercomprises a pressure controlled switch valve (38) in circuit via thepressure limiting valve device (26) with a second, piston-rod-side,cylinder chamber (12) of the work cylinder (4), and a switch valve(58)in circuit via the pressure limiting valve device (26) with the firstcylinder chamber (10) of the work cylinder (4); andwherein a signalpressure for control of the switch valve (38) is provided for adjustmentof pressure of the second cylinder chamber (12), the switch valve (58)serving also to direct pressure of the first cylinder chamber (10) to apressure relief section of the damping device (26) during the forwardclosing stroke, whereby the damping device serves for applying a dampingpressure which counteracts a sudden downward movement upon cutting ofthe workpiece (W) during the work stroke of the work piston (2).
 2. Ahydraulic control according to claim 1, wherein the damping device has apressure limiting valve device (26) via which, upon the exceeding of amaximum damping pressure, the hydraulic fluid in the return line (22)can be relieved into tank (T).
 3. A hydraulic control for a dividingmachine tool, suitable for a shear or cutting press, a work cylinder ofwhich can be controlled via a valve device so that hydraulic fluid canbe fed to a work piston for a work stroke, for a forward closing strokeand for return movement away from a workpiece, wherein a back pressurevalve device is provided in order to control a closing stroke of thework piston against a predetermined back pressure, in the manner that aconnection is opened and closed between a tank of the hydraulic systemand a return line from the work cylinder, wherein there is provided adamping device with a pressure limiting valve device (26), the workcylinder (4) having a first, piston side, chamber (10), and thehydraulic control further comprises a pressure controlled switch valve(38) in circuit via the pressure limiting valve device (26) with asecond, piston-rod-side, cylinder chamber (12) of the work cylinder (4),and a switch valve(58) in circuit via the pressure limiting valve device(26) with the first cylinder chamber (10) of the work cylinder (4);andwherein a signal pressure for control of the switch valve (38) isprovided for adjustment of pressure of the second cylinder chamber (12),the switch valve (58) serving also to direct pressure of the firstcylinder chamber (10) to a pressure relief section of the damping device(26) during the forward closing stroke, whereby the damping deviceserves for applying a damping pressure which counteracts a suddendownward movement upon cutting of the workpiece (W) during the workstroke of the work piston (2); wherein the damping device has a pressurelimiting valve device (26) via which, upon the exceeding of a maximumdamping pressure, the hydraulic fluid in the return line (22) can berelieved into tank (T); and wherein for the limiting of the backpressure, a control pressure which can be limited via a back pressurevalve device (54) can be applied to a control side of thepressure-limiting valve device (26), and, for the damping, a dampingcontrol pressure which is greater than the control pressure can beapplied on the control side.
 4. A hydraulic control according to claim3, wherein the damping control pressure which branches off from a feedline (24) to the work cylinder (4) is fed via a bypass line (68), and aflow valve (70) and a non-return valve (72) are connected one behind theother therein.
 5. A hydraulic control according to claim 2, wherein thecontrol pressure can be relieved by the pressure controlled switch valve(38) into the tank (T) and a control side of which can be connected withthe feed line (24) is connected in a control circuit of the pressurelimiting valve device (26).
 6. A hydraulic control for a dividingmachine tool, suitable for a shear or cutting press, a work cylinder ofwhich can be controlled via a valve device so that hydraulic fluid canbe fed to a work piston for a work stroke, for a forward closing strokeand for return movement away from a workpiece, wherein a back pressurevalve device is provided in order to control a closing stroke of thework piston against a predetermined back pressure, in the manner that aconnection is opened and closed between a tank of the hydraulic systemand a return line from the work cylinder, wherein there is provided adamping device with a pressure limiting valve device (26), the workcylinder (4) having a first, piston side, chamber (10), and thehydraulic control further comprises a pressure controlled switch valve(38) in circuit via the pressure limiting valve device (26) with asecond, piston-side, cylinder chamber (12) of the work cylinder (4), anda switch valve (58) in circuit via the pressure limiting valve device(26) with the first cylinder chamber (10) of the work cylinder (4);andwherein a signal pressure for control of the switch valve (38) isprovided for adjustment of pressure of the second cylinder chamber (12),the switch valve (58) serving also to direct pressure of the firstcylinder chamber (10) to a pressure relief section of the damping device(26) during the forward closing stroke, whereby the damping deviceserves for applying a damping pressure which counteracts a suddendownward movement upon cutting of the workpiece (W) during the workstroke of the work piston (2); wherein the damping device has a pressurelimiting valve device (26) via which, upon the exceeding of a maximumdamping pressure, the hydraulic fluid in the return line (22) can berelieved into tank (T); and wherein the hydraulic control furthercomprises a flow valve (46) provided in the control circuit downstreamof the back pressure valve device (54).
 7. A hydraulic control accordingto claim 3, wherein a control line (34) which conducts the controlpressure branches off from the return line (22) and has, in thebranching region, another control valve (36), preferably a nozzle.
 8. Ahydraulic control according to claim 7, wherein the bypass line (68)debouches into the control line (34) downstream of said further flowvalve (36).
 9. A hydraulic control according to claim 3, wherein thepressure limiting valve device (26) has a power part (28) which isprestressed into its closed position by the action of the controlpressure and/or of the damping control pressure and spring action.
 10. Ahydraulic control according to claim 3, further comprising amaximum-pressure limiting valve (50) by which the pressure in thecontrol circuit can be limited to a maximum values provided in thecontrol circuit.
 11. A hydraulic control according to claim 4, whereinthe pressure limiting valve device (26) has a power part (28) which isprestressed into its closed position by the action of the controlpressure and/or of the damping control pressure and spring action.
 12. Ahydraulic control according to claim 4, further comprising amaximum-pressure limiting valve (50) by which the pressure in thecontrol circuit can be limited to a maximum value, provided in thecontrol circuit.
 13. A hydraulic control according to claim 4, whereinthe flow valve (70) is a nozzle.
 14. A hydraulic control according toclaim 6, wherein the flow valve (46) is a nozzle.
 15. A hydrauliccontrol according to claim 7, wherein the control valve (36) is anozzle.
 16. Hydraulic control for a dividing machine tool, suitable fora shear or a cutting press, with a work cylinder (4) that can be movedin one direction in a closing stroke and a work stroke, and in theopposite direction in a return stroke, said work cylinder having a firstcylinder space (10) on the piston side and a second cylinder space (12)on the piston rod side, wherein pressure medium can be forced from thesecond cylinder space (12) through a return line (22) into a tank (T)with a pilot damping device (26) incorporated into return line(22);wherein the pilot damping device (26) connects with a main valveelement (28) upon which, in an opening direction, the pressure in thesecond cylinder space (12) acts and in the closing direction a controlpressure present in a control chamber (31) of the main valve element(28) acts; wherein said control chamber (31) is in a control circuit,said control chamber (31) being capable of being impacted by a controlpressure that corresponds to a retaining pressure in the second cylinderchamber (12) and at a control pressure limiting valve (54) of thedamping device (26) to which, during the closing stroke, control oilflows from the return line (22) through a control line (34) thatbranches off return line (22); wherein the hydraulic control includes apilot valve (38) and a non-return valve (72), and said control chamber(31) is capable of being relieved of the control pressure during thework stroke through the pilot valve (38) controlled by the pressure inthe first cylinder space (10), and said control chamber (31), at the endof the work stroke, is capable of being exposed to a control pressurethat corresponds to a damping pressure in said second cylinder space(12); wherein the control pressure, corresponding to the dampingpressure, is capable of being increased by an influx of control oil fromthe first cylinder space (10) through a bypass line (68) into thecontrol chamber (31), and with the non-return valve (72) that has ablocking action on the first cylinder space (10) being provided in thebypass line (68).
 17. A hydraulic control according to claim 16, whereina nozzle (70) is located in bypass line (68) that is in series with thenon-return valve (72).
 18. A hydraulic control according to claim 16,wherein a control side of pilot valve (38) is controlled by the pressurein the first cylinder space (10) and can be connected with a supply line(24) that leads to the first cylinder space (10).
 19. A hydrauliccontrol according to claim 16, wherein a nozzle (46) is located in thecontrol circuit between the inlet of the control pressure limiting valve(54) and the control chamber (31).
 20. A hydraulic control according toclaim 16, wherein a nozzle (36) is located in the control line (34) thatbranches off return line (22).
 21. A hydraulic control according toclaim 20, wherein the bypass line (68) debouches into the control line(34) downstream of said flow valve (36).
 22. A hydraulic controlaccording to claim 21, wherein the pilot valve (38) that is controlledby the pressure in the first cylinder space (10) is a switching valvethat connects the control chamber (31) in a first switching positionwith a portion of the control line (34) that is located downstream fromthe inlet of the bypass line (68) and in a second closed positionconnects the control chamber (31) by a relief line (42).
 23. A hydrauliccontrol according to claim 16, wherein the main valve element (28) isbiased by a spring (66) into its closed position.
 24. A hydrauliccontrol according to claim 16, wherein a second control pressurelimiting valve (50) is located in the control circuit (31), by whichvalve the control pressure can be limited to a value that is above thevalue for which the first-mentioned control pressure limiting valve (54)is set.
 25. A hydraulic control according to claim 16, wherein anadditional pilot valve (58) is provided in the control circuit and isdesigned as a switching valve, said additional pilot valve (58)connecting the outlet of control pressure limiting valve (54) with arelief line (64) in a first switch position, and shutting it off in asecond switch position assumed during the return stroke of work cylinder(4).
 26. A hydraulic control according to claim 25, wherein a controlside of pilot valve (38) that is controlled by the pressure in the firstcylinder space (10) is connected in the first switch position of theadditional pilot valve (58) with a feed line (24) that leads to thefirst cylinder space (10) and in the second switch position of theadditional pilot valve (58) is connected with a relief line (64).